After spending his entire life dealing with a painful muscle disorder, 13-year-old Kaleo Niko made the difficult decision to have both legs amputated below the knee.

Niko was born with club feet and arthrogryposis , a congenital disorder in which the muscles and tendons do not develop naturally. Rather than normal muscle tissue in his calves, Niko's muscles were instead fibrous and fatty, which resulted in an inability to walk easily and put immense pressure on his joints.

Multiple surgeries throughout his life were able to fix the shape of his feet, but the surgeries combined with the disorder left him in immense pain and unable to move his feet properly.

Niko likened the feeling to having "stabbing pain" in his legs, when he walked.

Dr. Teresa Hennessey, a pediatric orthopedic surgeon who treated Niko at the Shriner's Children Hospital in Salt Lake City, said that Niko's case and his treatment were unique because the disorder was confined to the muscles below his knees and his elbows. Niko's legs above the knee are basically healthy and do not need any prostheses. The disorder can affect the hands, feet or even the entire body. Arthrogryposis in the arms can result in a limited range of motion in joints and muscle weakness.

Since arthrogryposis is not a degenerative disease, meaning it will not spread, Hennessey said after Niko recovers, he should be able to utilize prosthetics on his lower legs to walk and run.

"It was like he was walking on solid wood prosthetics … that were painful," said Hennessey of Niko's condition before the surgery. "This is life-changing for Kaleo. He is going to function better than he ever has."

Kaleo Niko, 13, chose to have his legs amputated after dealing with a rare muscular disorder. (KMGH/ABC News)

After making his decision, Niko had a cast made of his legs as a memento before the operation, but didn't regret his choice.

"I didn't really question it, I wanted it to go as good as it could go," he told KGMH-TV.

Although Niko is currently undergoing additional treatment for a possible infection in his leg as a result of the surgery, Hennessey said she expects the setback to be a minor one and that when he gets used to prosthetics he will be more mobile than ever before.

A few weeks after the surgery, Hennessey said Niko appeared to be dealing with the amputation without having any phantom limb symptoms or anxiety.

"He handled it so well. As a surgeon the moment [when they wake up] is the moment of truth," said Hennessey. "He woke up and pulled the blanket off his legs and said, 'That looks weird.'"